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The measuring accuracy of CMM (Coordinate Measuring Machine) is influenced by many factors, such as temperature, humidity, measuring force and method of signal acquisition. For thin parts, the influence of measuring force is especially obvious. In this paper, the relationship between measuring force and measurement accuracy is studied for a thin part with a U-shaped cross-section. By analyzing the structure of the probe and establishing the force model, the influencing factors of the accuracy of CMM are obtained, and the influence of the contact deformation and the bending deformation on the measurement accuracy is analyzed from the point of view of material mechanics. At the same time, the measurement accuracy of different measuring cross-sections is analyzed. Through the research of this paper, the relationship between measuring force and CMM is established, and an effective method to improve the accuracy of CMM is also found. 1 Structural analysis of trigger probe and error principle The probe is an important part of CMM, consisting of a measuring rod and a ruby head, which is mainly used to measure the working surface, move through the mechanical position of the measuring end, trigger the signal and collect a measurement data. The internal structure of the RDS XXT (Carl Zeiss’ products) probe is shown in Fig. 1, the rod is fitted on the ring and the outer wall of the ring has a uniform contact terminals. When the rod is not measured, the outer wall of the ring is kept in contact with the terminals. One contact detachment is caused at least when the spherical end of the measuring rod is contacted with the workpiece, thus causing the circuit to disconnect, produce step signal, through the sampling circuit, will be along three axes direction of the coordinate data storage processing. Fig. 1-measuring rod, 2-ring cage. Fig. 1 Schematic diagram of the head probe The process between the probe end and the measured part contacting to the probe trigger is "Pre-range" during the CMM is working. The Pre-range is not an error source, because a standard sphere with known radius is used to determine a reasonable probe radius to eliminate the effect of the average Pre-range during the probe certification process. The different triggering forces, that is the production of the Pre-range changes, as shown in Figure 2, the figure can be seen visible three convex angle, reflecting the amount of the change in the Prerange "δ". This mode shows symmetry of the probe, which greatly reduces the ability of CMM to perform high-precision measurement.

Influence of measuring force on CMM accuracy